Deammonification is a two-step process for biological treatment of ammonium-containing waters which combines partial nitritation and anaerobic ammonium oxidation (anammox). In the first step, aerobic ammonium oxidizing bacteria (“AOB”) convert about 50% of the incoming ammonia to nitrite. In the second step, anaerobic ammonium oxidizing bacteria (“AMX”) convert the remaining ammonium and nitrite to nitrogen gas and a small amount of nitrate. This reaction can take place in two separate reactors, with partial nitritation occurring in an aerobic reactor followed by anammox occurring in an anoxic reactor (see e.g., U.S. Pat. No. 6,485,646 B1), or it can take place in a single reactor. A number of single reactor configurations exist including upflow granular sludge, moving bed biofilm reactor (MBBR), and sequencing batch reactor (“SBR”) with biomass separation device (see e.g., U.S. Patent Application Publication No. US2011/0198284 A1). Deammonification provides an advantage over traditional nitrification-denitrification in that it consumes 100% less organic carbon, produces 90% less sludge and consumes 60% less oxygen.
The deammonification “MBBR” process consists of a continuously stirred-tank reactor containing buoyant free-moving plastic biofilm carriers kept in suspension in the bulk liquid by aeration or mechanical mixing. The conversion of ammonium takes place in a biofilm attached to the plastic biofilm carriers in which AOB exist on the exterior of the biofilm, while AMX exist deeper within the biofilm in an anoxic environment.
The key parameters for control of this process are influent flow and dissolved oxygen (“DO”) concentration. Flow of water to the reactor determines the ammonium load on the system as well as incoming alkalinity. It is desirable to maintain a low dissolved oxygen concentration (e.g., <2 mg/L) in the reactor to limit the potential growth of nitrite oxidizing bacteria (“NOB”) and to avoid inhibition of AMX by the diffusion of oxygen into the anoxic zone of the biofilm. The DO concentration in the reactor is determined by airflow to the reactor, biological activity in the reactor, and temperature. Alkalinity is consumed by the bacteria to complete ammonium oxidation. If the alkalinity consumed by the bacteria is greater than the influent alkalinity, then the pH in the reactor will decrease. If alkalinity consumed by the bacteria is less than the influent alkalinity, then the pH in the reactor will increase.
A deammonification MBBR process can be operated with intermittent aeration. See, e.g., Zubrowska-Sudol, M., Yang, J., Trela, J., Plaza, E., “Evaluation of deammonification process performance at different aeration strategies,” published in Water Science and Technology. 63(6), 1168-1176 (2011); and Ling D., “Experience from commissioning of full-scale DeAmmon™ plant at Himmerfiarden (Stockholm),” published in 2nd IWA Specialized Conference on Nutrient Management in Wastewater Treatment Processes (2009). However, continuous aeration is preferred due to simplicity of operation, more accurate readings of online signals, and elimination of the need for mechanical mixing during non-aerated phases. Online measurements from probes located in the reactor or in the effluent can be used for monitoring performance of the process. This includes some combination of the following probes: pH, specific conductivity, ammonium concentration, nitrate concentration, nitrite concentration, or dissolved oxygen concentration. In addition an air flow meter in combination with an air flow control valve modulates airflow to the reactor based on signals from one or more of the aforementioned probes. This could also be achieved by a dedicated blower that is controlled to deliver a target air flow rate. The reactor cannot be operated without some form of aeration control due to the possibility of over-aeration leading to the accumulation of nitrite which is irreversibly inhibitory to AMX at high concentrations.
It is known that pH, conductivity, and DO sensors can be used to determine the intermittent air ON and OFF cycles in an intermittently aerated SBR (see, e.g., U.S. Pat. Nos. 7,846,334 B2 and 8,298,422 B2). It is also known that DO based aeration control can be used in a deammonification MBBR process (see e.g., U.S. Patent Application Publication No. US2013/0256217 A1 and U.S. Pat. No. 8,057,673 B2).
U.S. Pat. No. 7,846,334 B2 describes a method for treating ammonium-containing water in an intermittently aerated deammonification SBR in which the length of the aerated and non-aerated phases is controlled by a low and high pH setpoint. See also Wett, “Development and implementation of a robust deammonification process,” published in Water Science and Technology, 56 (7) 81-88 (2007). This method is specific to an intermittently fed, intermittently aerated SBR with the fluctuation of the range of pH values being at most 0.05 and the DO concentration being kept between 0.2 mg/L and 0.4 mg/L.
U.S. Pat. No. 8,298,422 B2 describes a method for treating ammonium-containing water in an intermittently aerated deammonification SBR in which a conductivity and/or DO concentration in the reactor determines the length of the aerated and non-aerated phases.
Joss, A., Siegrist, H., Salzgeber, D., Eugster, J., König, R., Rottermann, K., Burger, S., Fabijan, P., Leumann, S. & Mohn, J., “Full-scale nitrogen removal from digester liquid with partial nitritation and anammox in one SBR,” published in Environmental Science & Technology, 43(14), 5301-5306 (2009) describes a method for treating ammonium-containing water in a continuously or intermittently aerated deammonification SBR in which a conductivity or ammonia setpoint determines the end of the reaction phase of the SBR. In this method the conductivity or ammonia signal is not controlling the aeration but rather the length of the overall SBR cycle.
U.S. Patent Application Publication No. US2013/0256217 A1 describes a method for treating ammonium-containing water in a deammonification MBBR in which a DO setpoint is periodically adjusted by the controller based on ammonia removal and nitrate production ratios in the reactor. The ratios are calculated from sensor values in the tank and the DO setpoint is incrementally increased or decreased if the ratios fall outside of an optimal zone. A goal of this method may be to maximize ammonia removal by increasing the DO setpoint until an optimal ammonia removal percentage is met. However this method does not protect against running out of alkalinity in the reactor resulting in a dramatic decrease in pH. If the DO concentration setpoint is too high, then the pH will continue to decrease until all of the incoming alkalinity is consumed.
A key to the operation of deammonification reactors is the inhibition of nitrite oxidizing bacteria (“NOB”) that compete with anammox for substrate and for space within the biofilm. Strategies for inhibition of NOB include high free ammonia concentration, low dissolved oxygen concentration, high temperature, and transient anoxia. The method described in U.S. Patent Application Publication No. US2013/0256217 A1 aims to limit NOB growth by using a controller to decrease the DO setpoint when the nitrate production ratio is above the value that would be expected to be produced by AMX alone. If nitrate production is higher than 10-15% (indicating proliferation of NOB), then the process DO is limited in an effort to control NOB activity at the expense of losing NH4 removal.
U.S. Pat. No. 8,057,673 B2 describes a method for treating ammonium containing water in a two-reactor deammonification system in which partial nitritation takes place in the first reactor and anammox takes place in the second reactor. The first reactor is aerated to meet a DO setpoint between 0.5 mg/L and 1 mg/L. The pH in the first reactor is controlled to be between 7.5 and 8. In this method, the pH signal is not used to control aeration, but, instead, it is used to control the pH with the intent of inhibiting NOB in the aerobic reactor.
U.S. Pat. No. 8,268,173 B2 describes a method for controlling aeration in an integrated fixed film activated sludge (“IFAS”) process based on DO and ammonia concentration to account for variations in the amount of nitrifying biomass on the carriers versus the amount of nitrifying biomass in the mixed liquor. This method does not refer to a deammonification IFAS process (AOB in the mixed liquor and AMX on the carriers) but rather a process in which nitrification (AOB and NOB) takes place on both the carriers and in the mixed liquor.